Polymer systems results

New kinds of living polymer systems result from the reactions of transition metals with cyclic, strained olefins 16). These polymerizations proceed through the intermediacy of metal carbenes and are exemplified by the polymerization of norbomene initiated by bis(cyclopentadienyl)-titane-cyclobutane described recently by Grubbs17>. 

The application of the Maier-Saupe theory to the polymer system results in the nematic to isotropic (N-I) transition temperature, Tc, the order... 

The impact modification by the addition of elastomer traditionally decreases the stifhiess of a polymer system resulting in lower flexural modulus. Loss in flexural modulus with an increase in elastomer level was observed in all elastomer modified PP blends. The impact modification of PP by the addition of jS-nucleator does not reduce the stifhiess and ultimate strength of PP. However, their impact strength at low temperature is unsatisfatory, as the glass transition temperature of a and p PP which determine the low temperature brittleness are identical. 

Unlike traditional polymer composites containing micron-scale fillers, the incorporation of nanoscale CNTs into a polymer system results in very short distance between the fillers, thus the properties of composites can be largely modified even at an extremely low content of filler. For example, the electrical conductivity of CNT/epoxy nanocomposites can be enhanced several orders of magnitude with less than 0.5 wt.% of CNTs... 

The complexity of polymeric systems make tire development of an analytical model to predict tlieir stmctural and dynamical properties difficult. Therefore, numerical computer simulations of polymers are widely used to bridge tire gap between tire tlieoretical concepts and the experimental results. Computer simulations can also help tire prediction of material properties and provide detailed insights into tire behaviour of polymer systems. A simulation is based on two elements a more or less detailed model of tire polymer and a related force field which allows tire calculation of tire energy and tire motion of tire system using molecular mechanisms, molecular dynamics, or Monte Carlo teclmiques 1631. 

When polymers or other water-soluble substances are present in the sample, it is advantageous to add a small amount of chloroform to the initial reaction mixture after the subsequent addition of water, a two-phase system results which may be titrated in the usual way to a starch end point or by observing the disappearance of the iodine colour in the chloroform layer. 

The assessment of the contribution of a product to the fire severity and the resulting hazard to people and property combines appropriate product flammabihty data, descriptions of the building and occupants, and computer software that includes the dynamics and chemistry of fires. This type of assessment offers benefits not available from stand-alone test methods quantitative appraisal of the incremental impact on fire safety of changes in a product appraisal of the use of a given material in a number of products and appraisal of the differing impacts of a product in different buildings and occupancies. One method, HAZARD I (11), has been used to determine that several commonly used fire-retardant—polymer systems reduced the overall fire hazard compared to similar nonfire retarded formulations (12). 

Whea there are reactants with three or more functionahties participating ia the polymerization, branching and the formation of iatermolecular linkages, ie, cross-linking of the polymer chains, become definite possibiUties. If extensive cross-linking occurs in a polymer system to form network stmctures, the mobiUty of the polymer chains is greatiy restricted. Then the system loses its fluidity and transforms from a moderately viscous Hquid to a gelled material with infinite viscosity. The experimental results of several such reaction systems are collected in Table 6. 

Bromine- and iodine-containing fluoroolefins have been copolymerized with the above monomers in order to allow peroxide cure (14—21). The peroxide cure system does not requite dehydrofluorination of the polymer backbone, resulting in an elastomer that shows improved properties after heat and fluid aging. 

Addition cure silicones can be delivered from solvent, waterborne emulsions, or 100% solids systems. The solvent free versions employ base polymers of intermediate molecular weight to achieve processable viscosity. These base polymers can have reactive moieties in terminal and/or pendant positions. These lower molecular weight, more functional systems result in a tighter crosslink network which feels rubbery to the hand. Low amounts of high molecular weight additives are included in some formulations to provide a more slippery feel [51,52]. 

Usually, the transition of polymer systems into the oriented state occurs as a result of deformation e.g. upon exposure to external stress. When the polymers undergo deformation both the macromolecule as a whole and its parts (segments) can undergo orientation. The rates of these orientation processes are very different and, hence, the orienting forces affect first of all the orientation of chain segments and subsequently that of a chain molecule as a whole. However, by varying the extension velocity and the temperature, only the overall orientation process may predominate, thus extension of all chains occurs in a single act. 

Hence, the transition of a polymer system into the oriented state is a result of the competition of two fundamental properties of a polymer molecule (1) its inherent anisotropy which is the main reason for the ability of polymer systems to form an oriented phase and (2) its flexibility which favours coiling of a long molecule. The result of this competition is determined by the chemical nature of the molecule however, kinetic hindrance can prevent the transition into the oriented state. 

Thermal-oxidative decomposition has been studied by Beachell and Nemphos (B5), Grassie and Weir (G2), Notely (N3), Parker (PI), and Ryan (R4) for a variety of polymer systems. The principal results of these studies are ... 

Clays have long been used as fillers in polymer systems because of low cost and the improved mechanical properties of the resulting polymer composites. If all other parameters are equal, the efficiency of a filler to improve the physical and mechanical properties of a polymer system is sensitive to its degree of dispersion in the polymer matrix (Krishnamoorti et ah, 1996). In the early 1990s, Toyota researchers (Okada et ah, 1990) discovered that treatment of montmorillonite (MMT) with amino acids allowed dispersion of the individual 1 nm thick silicate layers of the clay scale in polyamide on a molecular. Their hybrid material showed major improvements in physical and mechanical properties even at very low clay content (1.6 vol %). Since then, many researchers have performed investigations in the new field of polymer nano-composites. This has lead to further developments in the range of materials and synthesizing methods available. 

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